One of the more rapidly growing areas of chemistry research involves ionic liquids (ILs) and room temperature ionic liquids (RTILs). The wide range of possible cation and anion combinations allows for a large variety of tunable interactions and applications. The uses and applications of RTILs have traversed many areas of chemistry and even areas of biochemistry. Reported thermal stability ranges of 300° C. in some cases, their ability to solubilize a variety of disparate molecules, and the fact that ionic liquids can be synthesized that are immiscible with both water and nonpolar organic solvents further add to their usefulness. While much work involving RTILs deals with their use as “green” solvents in organic synthesis, their characterization and the understanding of their unique physico-chemical and solvation properties are important areas of ongoing investigation. Some research in the field of ionic liquids has explored their fundamental properties in hopes that it would become apparent which cation-anion combinations give rise to specific and/or desired qualities. Thus far, this approach has met with only limited success.
Early work seemed to indicate that the anionic constituents of ionic liquids may have a greater influence on their physical and chemical properties. However, this notion may be due, in part, to the fact that the ionic liquids studied contained not only a variety of different anions, but closely related, structurally similar cations. Indeed, anions such as halides possess higher hydrogen bond basicity character (Cl>Br>I) and readily hydrogen bond to generally form viscous liquids. This is not to say that only coordinating anions produce viscous liquids; it is well known that the viscosity of 1-alkyl-3-methylimidazolium ionic liquids is found to increase with increasing alkyl chain length even when paired with non-coordinating anions such as hexafluorophosphate (PF6−) and bis(trifluoromethylsulfonyl)imide (NTf2−). While the cation and its structure can certainly influence the surface tension, melting point, viscosity, density, and thermal stability as well as interact via dipolar, π-π, and eta-π interactions with dissolved molecules, its range of effects has not been studied as extensively as it has for anions.
Despite their touted stability, many of the more common ionic liquids are susceptible to chemical and thermal degradation. Recently, it was reported that when 1-butyl-3-methylimidazolium chloride (BMIM-Cl) is exposed to the atmosphere and heated, it begins to turn from a pale yellow to amber color at 120° C. When heated further, the ionic liquid begins to show obvious signs of decomposition at and above 150° C. Most recently, a new class of “high stability ionic liquids” based on bulky cations and triflate anions was introduced and it was reported that the robustness of some of the more traditional ionic liquids appears to be less than previously thought (in terms of both lower thermal stability and higher volatility). MacFarlane and co-workers reached similar conclusions via use of the ‘step tangent method’ for thermogravimetric analysis (TGA) to more accurately determine degradation temperatures of imidazolium-based cations. They point out that significant evolution of volatile degradation products takes place well below previously reported degradation temperatures. A maximum operating temperature parameter was proposed to provide a more appropriate estimate of thermal stability using TGA.
Techniques of solid phase extraction and solid phase microextraction are known. Ionic liquids have been used in task-specific liquid-liquid extraction for use in extraction of Hg2+ and Cd2+ from water. U.S. Patent Publication No. 2006/0025598 reports the use of diionic liquid salts and immobilized ionic liquids for solid phase extraction. U.S. Pat. No. 6,531,241 reports cyclic delocalized cations joined together by spacer groups.